Thermoacoustic oscillations pose a risk for any type of combustion applications. They cause pressure fluctuations with a high amplitude, restrict the operating range, and may increase the emissions associated with combustion. These problems occur in particular in combustion systems with low acoustic attenuation, as often represented by modern gas turbines.
In standard combustors, the coolant air flowing into the combustor acts in a sound-absorbing manner and in this way contributes in the attenuation of thermoacoustic oscillations. In order to achieve low NO.sub.x emissions, an increasing share of the air is conducted through the burners themselves in modern gas turbines, and the coolant air stream is reduced. The sound absorption associated with this causes the initially mentioned problems to occur more often in modern combustors.
One method of absorbing the sound consists of connecting Helmholtz dampers inside the combustor hood or near the coolant air supply. But given the tight space conditions typically found with modern combustors built in a compact manner, the arrangement of such dampers may be difficult, however, and may be associated with high constructive expenditure.
Another possibility is the control of thermoacoustic oscillations with active acoustic excitation. The shear layer that develops near the burner is hereby stimulated acoustically. With a suitable phase position between the thermoacoustic oscillations and the excitation, an absorption of the combustor oscillations can be achieved. Such a solution requires the attachment of additional elements near the combustor, however.